Radiation detection phosphor



Sept. Z3, 1958 R. c. Fox ETAL RADIATIQN DETEcTroN PHosFHoR Filed July25. 1951 0 snm@ ROS Ovnu TOL NF WCS f myn. om

l ATTORNEY United States Patent RADIATION DETECTION PHOSPHOR Roy C. Fox,Pasadena, and Walter S. Lusby, Severua Park, Md., assignors toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvama Application July 25, 1951, Serial No. 238,482

7 Claims. (Cl. Z50-71) Our invention relates to the detection ofradiations and, more particularly, to phosphors for the detection ofradiations.

In accordance with the prior art of which we are aware, radiationdetector phosphors have been used which constitute a plurality of smallcrystals of a material which is sensitive to high-frequency radiation,such as gamma rays, impinging thereon and which is capable of emittinglight in response to the actionof the impinging radiation. However, ithas been found that some radiations, such as gamma rays, have a highpenetrating power, and a large volume of the crystals is required inorder to absorb a substantial percentage `of the gamma rays impingingthereon. It has, accordingly, been the practice to build a large crystalof the material sensitive to the radiation to be detected, which issubstantially transparent to the light emitted by that material inresponse to radiation impinging thereon. A large crystal built in thismanner provides a larger volume of phosphor so as to absorb a largepercent of the radiation impinging thereon while allowing a largepercent of the light emitted by the material to escape from thatmaterial. However, it has been found to be highly diicult and expensiveto build a large crystal of most of the desirable phosphors.

It is, accordingly, van object of our invention to build an improvedradiation detector.

Another object of our invention is to provide an improved phosphor for aradiation detector.

An ancillary object of our invention is to provide a material which willabsorb a large percent of the radiation impinging thereon, while beingsubstantially transparent to the radiation emitted by that material inresponse to said radiation.

Still Vanother object of our invention is to provide a material having alarge volume which has characteristics similar to those of a largesingle crystal phosphor but which is substantially less expensive.

ln accordance with our invention, we provide a plurality of smallcrystals sensitive to the radiation to be detected and capable ofemitting light in response thereto. The crystals are embedded in amaterial having the same index of refraction as the crystal, and thematerials are joined in such a manner that the crystals and the materialaround them form a translucent body. This material comprising thecrystals and their supporting material is surrounded by alight-reflecting coating except NVice 2 embodying our inventionemploying 'a `liquid supporting material, and

Figure 2 is a showing in cross-section of a phosphor in accordance withour invention in which a solid supporting material is employed.

In accordance with one embodiment of our invention, we provide acontainer 2 having an outer layer or Wall 4 and an inner layer or wall6. The layers 4 and 6 comprising the container wall are made of amaterial which is transparent to the radiation to be detected. The innerlayer 6, in addition to being transparent to the radiation to bedetected, is made of a light reflecting material. Thus, for example, ifthe radiation to be detected is gamma rays, an outer wall 4 of a thinlayer of aluminum would be suitable. While the container is shown ashaving two layers, it is nevertheless understood that a wall of a singlelayer could be built which would be suitable for most purposes. Insidethe container 2 is provided a plurality of small crystals 8 of aphosphor material such as calcium tungstate, which are responsive togamma rays and are capable of emitting light in response to thatradiation. By using the term light, we do not mean to restrict thematerials to those emitting radiations in the visible range, but ratherwe mean to include those emitting radiations in the ultraviolet andinfrared ranges inasmuch as most photosensitive devices are responsiveto some radiations in these frequency ranges. Surrounding the crystals 8in the container 2 is a supporting material 10 having an index ofrefraction substantially equal to the index of refraction of thecrystals 8 for the light emitted by the crystals 8 in response to theaction of the radiation to be detected impinging on those crystals 8. ByWay of example, methylene iodide would be a suitable supporting materialfor use with a phosphor of calcium tungstate.

In the preferred embodiment of our invention, the supporting material 10is a liquid which is capable of wetting the surfaces ofthe crystals 3without being capable of dissolving the crystals 8. By saying that theliquid is not capable of dissolving the crystals 8, we do not mean thatthe liquid cannot contain therein a solvent of the crystals 8 because itis understood that the liquid 10 might comprise a solvent of thecrystals 8 which is already saturated with the material of the crystals8.

In accordance with the broader aspects of our invention, the supportingmaterial l0 surrounding the crystals 8 need not be a liquid but couldinstead bea solid 11 as shown in Fig. 2. The solid has an index ofrefraction which is close to the index of refraction of the crystals 8provided the crystals are immersed in the supporting material, i. e.,provided the junction of the surfaces of the crystals S and thesupporting solid 10 were such as to produce only a small amount ofdiffusion. This might be accomplished by mixing the crystals and thesupporting material while the latter is a liquid and then causing thesupporting material to solidify. An example of such a solid combinationwould be anthracine crystals in polyester resin. If the supportingmaterial 1l is a solid, it is, of course, possible that in somesituations the outer layer 4 of the wall could be omitted and thereflecting layer 6 could be coated on the supporting material. As shownin Fig. 2, the phosphor combination 8, 11 could be ernployed without anyreflecting layer. Imbedding the crystals in a solid has the additionaladvantages of making the region moistureproof and dustproof andprotecting the crystals from shock.

In some situations, it may be desirable to use a phosphor which issensitive to the radiation to be detected but which does not emit lightto which the light-responsive apparatus is responsive. In such a case,it would be desirable to employ a plurality of phosphors one of whichis4 sensitive to the ,radiation to V,be detected and the other of whichis sensitive to the radiation emitted by the rst and capable of emittingradiation to which the detecting apparatus istsensitive.

By using a plurality of phosphors, it is alsopossible to c ause -theresponse 4'of the detectorlto be proportional to energy, i. e., to causethe response to a given energy of short wavelength radiation to be Ymorenearly equal to the responseto the .same quantity ofenergy oflongWavelengthi In accordance withfthe broader aspects of our invention,thesnpporting material `could be a gas. ndices ofnrefraction of a solidand a gas could be reasonably well matched `in many casesby placinggthegas under several atmospheres pressure.

In 911, Wallof'the crystal container is a small opening 1,2whichis'transparent to ythe radiation emitted by the phosphor crystals8. The openingy 12 in the wall of the crystal container 2 is locatedl soas `to be opposite the photo-cathode 14 of afphoto-multiplier tube 16 orother light-responsive apparatus. Light emitted by the crystals 8 isthusreflected aroundV the inside of the crystal container by thereectinglayer 6 until it reaches the transparentopening 12 inthe crystalcontainer 2 and leaves the crystalrcontainer therethrough. Light leavingthe crystal containerrthrough-the transparent opening therein impingesonthe photo-cathode 14 of the photo-multiplier tube 16, causingelectrons to be emitted from the photocathode 14. The Velectrons arethen employed in a manner well known in the art to give an indication ofthe light photons impinging on the photo-cathode.

We have thus described a radiation detector comprising a phosphor bodywherein substantially all of the gamma rays impinging thereon may beabsorbed therein, and substantially al1 of the light produced therein inrespouseto gamma rays impinging thereon is emitted therefrom. Thisphosphor body is easy to construct and may readily be constructed in amuch larger and more eicient size than was possible with the devices ofthe prior art.

Although We have shown and described specic embodiments of ourinvention, we are aware that` other modiiications thereof are possible.Our invention, therefore, is-not to be restricted except insofar as `isnecessitated by the .prior art and the spirit of the invention.

We claim as our invention:

. 1. A scintillation counter comprising a unitary radiation-responsivedevice including a plurality of crystals adapted to translate theincoming radiation into light impulses, said crystals beingsubstantially transparent to light impulses developed therewithin, ahomogeneous substance surrounding said crystals, said substance beingsubstantially transparent to light impulses developed withinsaidcrystals and having a refractive index substantially equal to therefractive index of said crystals, and a detector positioned adjacentsaid device for translating lsaid light impulses into electricalcurrent.

2..A counter for measuring radiation comprising, a container, asubstantially transparent fluid within said container, a plurality ofcrystals submerged in and surrounded by said fluid, said crystals beingsubstantially transparent to light impulses developed therewithin andbeing of a material to interact with incoming radiation v particles toproduce a plurality of photons of light that emerge substantiallysimultaneously as a result of said interaction, said uidhaving arefractive index substantially the same as the refractive index of saidcrystals, and a photosensitive means responsive to said light emergence.

3. A phosphor for detecting radiation comprising: a plurality of smallcrystals of a material sensitive -to the radiationto be detected andcapable of emitting light in response to said radiation and beingsubstantially transparent to such emitted light, and a liquid which issubstantially transparent to such emitted light and has an index ofrefraction which is substantially the same as the index of refraction ofvsaid crystals, with said crystals being immersed in and surrounded bysaidliquid.

4. A phosphor for detecting radiation comprising: a plurality ofcrystals sensitive to the radiation to Vbe detected and capable ofemitting light in response thereto and being substantially transparentto such emitted light, and a solid material which is substantiallytransparent to such emitted light and has an index of refractionsubstantially equal to the index of refractionof saidy crystals, withsaid crystals being embedded in and surrounded by said solid.

5. A phosphor for detecting radiation comprising: a plurality of smallcrystals of a first material sensitive to radiation to be detected of afirst Wavelength andy capable to emitting a secondary radiation inresponse thereto and being substantially transparent to said secondaryradiation, a plurality of crystals of a second ymaterial sensitive tothe secondary radiation emitted by said crystals of ,said rst materialand capable of emitting light in Vresponse thereto and beingsubstantially transparent to said emitted light, and a material which issubstantially transparent to said emitted light and hasan index ofrefraction which is substantially the same as the index of refraction ofsaid last-mentioned crystals, with said crystals being immersed in andsurrounded by said material.

6. Apparatus as claimedv in claim 5 characterized in that thelast-mentioned material is a liquid.

7. Apparatus asclaimed in claim 5 characterized in that thelast-mentioned material is a solid.

. References Cited in the tile of this patent UNITED STATES PATENTS2,534,932 Sun Dec. 19, 1950 2,559,219 Ludeman July, 1951 2,573,200Husley i Oct. 30, 1951 OTHER REFERENCES Scintillation Counting WithSolutions, by Hartmut Kallmann, from Physical Review, volume 78, 1950,pp. 621-22. Y

